Python Altair Charts - Highlight points on a time series chart

Question

In Altair, is there a way to draw attention of the viewer to a particular point on a time-series chart?

For example, by drawing a circle around the data point:

and then allowing a custom text to be displayed in a tooltip?

Thank you!

Answer 1

You can do this with a layer chart; for example:

import altair as alt
import pandas as pd
import numpy as np

rng = np.random.default_rng(0)
data = pd.DataFrame({
  'x': pd.date_range('2021-01-01', freq='W', periods=52),
  'y': rng.normal(size=52).cumsum()
})

line = alt.Chart(data).mark_line().encode(
    x='x',
    y='y',
)

callout = alt.Chart(data.iloc[7:8]).mark_point(
    color='red', size=300, tooltip="Tooltip text here"
).encode(
    x='x',
    y='y'
)

line + callout